Ternary tin zinc alloy, electroplating solutions and galvanic method for producing ternary tin zinc alloy coatings

ABSTRACT

The invention relates to ternary tin zinc alloy coatings 30-65 wt. % tin, 30-65 wt. % zinc and 0.1-15 wt. % metal from the following group as a third alloy component; iron, cobalt, nickel. Correspondingly, alloy coatings can be produced by means of electrolytic deposition from aqueous galvanic electroplating solutions which contain the components of the alloy in a dissolved form. The alloy coatings are characterised in that they have a particularly high resistance to corrosion and are particularly suitable as anti-corrosion protective coatings on iron-based materials.

[0001] The invention concerns new ternary tin-zinc alloys of specific compositions which contain a metal from the group iron, cobalt, or nickel as a third alloy component. The invention further concerns electrolytic baths and a galvanic process for producing such ternary tin-zinc alloys, as well as their use as corrosion-protection layers or decorative layers.

[0002] It is well known that ferrous materials can be protected against corrosion by coating with zinc and subsequent passivation, such as by chromating (based on Cr⁺⁶) or chromiting (based on Cr⁺³), evident by a yellow, blue, black, or olive-green coloration of the surface. These measures can be used to attain protection times of 200 to 600 hours to the first appearance of red rust in salt mist testing (DIN 50021-SS) (Corrosion protection by protective layers and coatings, D. Grimme and J. Krüuger, Weka Fachverlag füur technische Führungskräafte, Augsburg).

[0003] More stringent requirements, such as resistance of up to 1000 hours until the first appearance of red rust in the salt mist test, can be met by coating with zinc alloys which contain nickel, cobalt or iron as components of the alloy and subsequent chromating. The proportions of the alloying elements can, for instance, be from less than 1% by weight, such as 0.4-0.6% by weight Fe in the ZnFe system up to 15% by weight, such as 12-15% by weight Ni in the ZnNi system (Zinc alloying processes: Properties and applications in technology, Dr. A. Jimenez, B. Kerle and H. Schmidt, Galvanotechnik 89 (1998)4).

[0004] Tin-zinc alloys can also be used as anticorrosion coatings for iron. Values of up to 1000 hours until the first appearance of red rust in salt mist testing are attained with chromated SnZn coatings. The most favorable alloy composition is 70% by weight Sn and 30% by weight Zn. The low hardness, only about 50 HV, of SnZn coatings is considered a disadvantage (Tin-Zinc Plating, E. Budmann and D. Stevens, Trans. IMF 76 (1998)3).

[0005] Observation of developments in the field of corrosion protection of ferrous materials, as in the automobile industry, indicates that there will be higher requirements for anticorrosion systems in the future, which cannot be met with known processes. Such increased requirements are on the order of 3000 hours resistance in salt mist tests. Furthermore, such anticorrosion coatings should have the highest possible hardness, be resistant to wear, and should also, as much as possible, be weldable.

[0006] The invention was, therefore, based on the objective of finding new alloy systems with particularly high corrosion resistance, and providing galvanic electrolytes for deposition of these alloys, to meet future requirements for anticorrosion effect.

[0007] Now it has been found that ternary tin-zinc alloys containing from 30 to 65% by weight tin, 30 to 65% by weight zinc, and 0.1 to 15% by weight of a metal from the group iron, cobalt, or nickel as the third alloy component meet these requirements superbly well.

[0008] The subject of the invention is, then, ternary tin-zinc alloys characterized in that they consist of 30 to 65% by weight tin, 30 to 65% by weight zinc, and 0.1 to 15% by weight of a metal from the group iron, cobalt, or nickel as the third alloy component.

[0009] The ternary tin-zinc alloys according to the invention preferably contain cobalt as the third alloying component.

[0010] Tin-zinc-cobalt alloys according to the invention preferably contain 40 to 55% by weight tin, 45 to 55% by weight zinc and 0.1 to 5% by weight cobalt. Tin-zinc-nickel alloys according to the invention preferably contain 35 to 50% by weight tin, 50 to 65% by weight zinc and 0.1 to 5% by weight nickel. Tin-zinc-iron alloys according to the invention preferably contain 40 to 55% by weight tin, 40 to 60% by weight zinc and 1 to 8% by weight iron.

[0011] The ternary tin-zinc alloys according to the invention can be produced from the individual components by fusion or powder metallurgy.

[0012] Electrolytic preparation is preferable, particularly with respect to typical applications. That is done by electrolytic deposition from aqueous galvanic electrolyte baths which contain the alloy components in dissolved form. The ternary tin-zinc alloys can be deposited onto substrates from alkaline, neutral, or weakly acidic electrolytic baths. Here an alkaline electrolyte is understood to be an electrolyte with a pH greater than 10. A neutral electrolyte is considered to be one with a pH from 6 to 10. A weakly acidic electrolyte is considered to be one with a pH of 3-6.

[0013] The alloy components are added to the aqueous electrolyte bath in the form of their compounds which are soluble and ionogenic in the particular medium. Tin is preferably added as the sulfate, chloride, sulfonate, or oxalate, or as sodium or potassium stannate. Zinc is preferably added as the sulfate, chloride, hydroxide, sulfonate or oxide. The element, iron, cobalt, or nickel, which acts as the third alloy component, is preferably added as the sulfate, chloride, hydroxide or carbonate.

[0014] The galvanic electrolyte according to the invention for producing ternary tin-zinc alloy coatings can also contain other additives common and well-known in plating technology. Those can be bases for pH adjustment, such as sodium, potassium, or ammonium hydroxide, or inorganic acids, such as hydrochloric acid, sulfuric acid, phosphoric acid, or boric acid; alkali salts of these acids as buffers and/or conductive salts; organic acids such as hydroxycarboxylic acids and/or their salts, such as citric acid; complexing agents such as EDTA, wetting agents, brighteners, etc. A person skilled in the art will be familiar with the criteria for qualitative and quantitative selection of such additives and their functions in galvanic baths.

[0015] The proportions of the metals in the electrodeposited alloy coating can be influenced, in the known manner, by the proportion of metals in the bath composition, by the nature and proportion of the other bath components, and by the deposition parameters.

[0016] For electrolytic deposition of the ternary tin-zinc alloys according to the invention, the substrate to be coated, such as a part made from a ferrous metal to be protected from corrosion, is immersed in an appropriate galvanic bath and connected as the cathode. The counterelectrodes can be anodes of insoluble or, preferably in the case of neutral or weakly acidic electrolytes, soluble materials. Insoluble anodes are usually of graphite or platinized titanium. It is convenient for soluble anodes to consist of the metals of the alloy to be deposited, preferably at the desired composition.

[0017] A temperature of about 20-70° C. and a current density of about 0.1-5 A/dm² are considered boundary conditions for deposition of the ternary tin-zinc alloys from the electrolytes according to the invention. Deposition rates of about 0.05-1 μm/minute are attained.

[0018] An alkaline electrolyte according to the invention can have the following typical ranges of compositions:

[0019] 10-50 g/l tin as the sulfate or chloride, or sodium or potassium stannate

[0020] 1-10 g/l zinc as the sulfate, chloride, hydroxide or oxide

[0021] 0.1-10 g/l cobalt, nickel, or iron as the sulfate

[0022] 1-20 g/l potassium or sodium hydroxide

[0023] 10-200 g/l of complexing agent

[0024] 0.1-10 g/l of a wetting agent

[0025] 0.1-5 g/l of a brightener

[0026] The galvanic deposition of the alloy is accomplished at temperatures in the range of 40-70° C. and at current densities of 1-5 A/dm² at deposition rates of 0.15-0.3 μm/minute. Graphite or platinized titanium can be used as anodes.

[0027] Organic acids and their salts, phosphonic acids, phosphonates, gluconates, glucoheptonic acids, glucoheptonates and ethylenediaminetetraacetic acid can be used as complexing agents. Surfactants, multifunctional alcohols, and betaines can be used as wetting agents and brighteners in the corresponding media.

[0028] The composition of the alloy layer can be varied by altering the proportions of the individual components in the bath. For instance, increasing the hydroxide content reduces the tin content, with corresponding increase of the other two metals in the coating.

[0029] Increasing the proportion of the complexing agent reduces the zinc content and increases the tin content in the coating. These changes have practically no effect on the third alloying metal.

[0030] A neutral electrolyte according to the invention can have the following typical ranges of compositions:

[0031] 10-40 g/l tin as the sulfate, or as sodium or potassium stannate

[0032] 0.5-10 g/l zinc as the sulfate, chloride, hydroxide or oxide

[0033] 0.1-10 g/l cobalt, nickel or iron as the sulfate, chloride, hydroxide or oxide

[0034] 50-200 g/l tetrasodium pyrophosphate

[0035] 1-20 g/l potassium hydroxide or sodium hydroxide

[0036] 10-200 g/l complexing agent

[0037] 0.1-10 g/l wetting agent

[0038] 0.1-5 g/l brightener

[0039] The electrolytic deposition of the alloy is accomplished at temperatures from 40 to 70 ° C., and at current densities of 0.5-3 A/dm², with deposition rates of 0.05-0.3 μm/minute. Graphite or platinized titanium are used as the anode. It is also possible to use soluble anodes.

[0040] The proportions in the alloy composition can be varied by varying the coating parameters.

[0041] A weakly acidic electrolyte according to the invention can have the following typical ranges of compositions:

[0042] 1-10 g/l tin as the sulfate or chloride

[0043] 1-10 g/l zinc as the sulfate, chloride, hydroxide or oxide

[0044] 1-20 g/l cobalt, nickel or iron as the sulfate, chloride, hydroxide or carbonate

[0045] 5-200 g/l of a carboxylic acid salt

[0046] 5-50 g/l of a buffer

[0047] 1-30 g/l sodium chloride

[0048] 1-20 g/l wetting agent

[0049] 0.1-5 g/l brightener

[0050] The electrolytic deposition of the alloy is accomplished at temperatures of 20 to 70° C. at current densities of 0.5-5 A/dm², with deposition rates of 0.1-1 μm/minute. Graphite or platinized titanium can be used as the anode. It is also possible to use soluble anodes. Boric acid, for example, can be used as the buffer.

[0051] The proportions in the alloy composition can be adjusted by changing the coating parameters (concentrations of the components in the solution, working parameters). For example, increasing the current density increases the proportion of zinc and nickel, cobalt or iron in the alloy and reduces the proportion of tin. Variation of the temperature in the specified range causes only insignificant changes of the composition of the alloy layer.

[0052] The ternary tin-zinc alloys according to the invention have very favorable material properties. On the basis of those properties, they can be used as an independent material, but also, especially, as coatings on substrates in various manners.

[0053] In general, the ternary tin-zinc alloys have particularly high resistance to corrosion, which is most strongly expressed in the SnZnNi and SnZnCo systems. Therefore those alloys are particularly suitable for anticorrosion layers on ferrous materials. Accordingly, the corresponding electrolyte solutions can be used preferentially to produce corrosionresistant layers on ferrous materials. Iron sheets, coated in this manner, combined with the usual passivation by chromating or chromiting, without other treatment, attain resistance to appearance of red rust of more than 3,000 hours.

[0054] Other advantageous properties can be controlled by selection of the specific third alloying element. The properties of the ternary tin-zinc alloy coatings according to the invention can be optimized, depending on the choice of the third alloying element. Table 1 shows an overview of the preferred third alloying element if either good corrosion resistance, hardness, wear resistance or weldability is desired. TABLE 1 Corrosion Hardness Wear Weldability SnZnNi + − + − SnZnFe − + − + SnZnCo + + − +

[0055] Of the three alloy systems, the SnZnFe and SnZnCo alloys attain the highest hardnesses. SnZnNi coatings have the highest resistances to wear. Such alloy coatings can, therefore, be used advantageously as wear-prevention layers in cases of mechanical stress. SnZnFe and SnZnCo coatings can be welded particularly well, and so are desirable as weldable coatings and contact surfaces in electronics. Table 2 shows the corresponding data for alloy systems selected as examples. TABLE 2 Coating SnZnNi SnZnFe SnZnCo Composition  Sn 44% Sn 52% Sn 46%  Zn 56% Zn 44% Zn 51% Ni 0.2%  Fe 4%  Co 3% Hardness 50 165 179 (HV 0.025) Wear 4.9 9.1 7.2 (mg weight lost per 1000 strokes according to Bosch-Weinmann) Weldability 0.3-0.4 0.8-1.2 0.3-0.6 (ZCT in seconds)

[0056] Aside from these areas of use determined by function, the ternary tin-zinc alloys according to the invention can also be used as final decorative coatings. For instance, the three alloy systems have interesting and appealing colors in the blue range, depending on the selection of the third alloying element.

EXAMPLE 1

[0057] An alkaline electrolyte for depositing an alloy consisting of 45% by weight Sn, 52% by weight Zn and 3% by weight cobalt has the following composition:

[0058] 30 g/l tin as sodium stannate

[0059] 2.4 g/l zinc as zinc oxide

[0060] 1 g/l cobalt as cobalt sulfate

[0061] 8 g/l potassium hydroxide

[0062] 50 g/l sodium citrate

[0063] 100 ml/l sodium phosphonate

[0064] 2.5 ml/l anionic surfactant

[0065] 1 g/l butynediol

[0066] This gives a pH of 11. The coating composition indicated above can be produced with this electrolyte at a temperature of 60° C. and current densities of 1-2 A/dm². In this case, about 0.2 μm of coating layer is built up per minute. The density of the alloy layer is 7.27 g/cm³.

[0067] Iron plates coated with this alloy at a thickness of 8 μm, after chromating (based on Cr⁺⁶) show the following resistance in the salt mist test of DIN 50021-SS:

[0068] First appearance of white rust in the time period 1800-3000 hours.

[0069] The test was terminated after 3000 hours, as no red rust had appeared by that time.

EXAMPLE 2

[0070] A neutral electrolyte for depositing an alloy consisting of 48% by weight tin, 49% by weight zinc and 3% by weight cobalt has the following composition:

[0071] 25 g/l tin as tin sulfate

[0072] 2.4 g/l zinc as zinc oxide

[0073] 1 g/l cobalt as cobalt sulfate

[0074] 130 g/l tetrasodium pyrophosphate

[0075] 2.5 ml/l anionic surfactant

[0076] 1 g/l butynediol

[0077] This solution has a pH of 8.5. The coating composition indicated above can be produced with this electrolyte at a temperature of 60° C. and current densities of 0.5-1 A/dm². 0.15 μm of coating is built up per minute. The density of the alloy layer is 7.27 g/cm³.

EXAMPLE 3

[0078] A weakly acidic electrolyte for depositing an alloy consisting of 49.2% by weight Sn,

[0079] 50.5% by weight Zn and 0.3% by weight nickel has the following composition:

[0080] 5 g/l tin as tin sulfate

[0081] 6.8 g/l zinc as zinc sulfate

[0082] 12 g/l nickel as nickel sulfate

[0083] 80 g/l sodium citrate

[0084] 25 g/l boric acid

[0085] 10 ml/l anionic surfactant

[0086] 1 ml/l beta-naphthol ethoxylate

[0087] This solution has a pH of 4.5. The coating composition indicated above can be produced with this electrolyte at a temperature of 40° C. and a current density of 1.5 A/dm². In this case, about 0.4 μm of the alloy layer is produced per minute. The density of the alloy layer is 7.2 g/cm³.

EXAMPLE 4

[0088] A weakly acidic electrolyte for depositing an alloy consisting of 52% by weight Sn, 44% by weight Zn, and 4% by weight iron has the following composition:

[0089] 5 g/l tin as tin sulfate

[0090] 6.8 g/l zinc as zinc sulfate

[0091] 10 g/l iron as iron sulfate

[0092] 80 g/l sodium citrate

[0093] 25 g/l boric acid

[0094] 10 ml/l anionic surfactant

[0095] 1 ml/l beta-naphthol ethoxylate

[0096] The pH of this solution is 4.4. This electrolyte can produce the layer composition stated above at a temperature of 40° C. and a current density of 1.5 A/dm². In this case, about 0.4 μm of the alloy layer is deposited per minute. The density of the alloy layer is 7.25 g/cm³. 

1. Ternary tin-zinc alloys characterized in that they consist of 30 to 65% by weight tin, 30 to 65% by weight zinc, and 0.1 to 15% by weight of a metal from the group iron, cobalt, or nickel as the third alloy component.
 2. Ternary tin-zinc alloys according to claim 1, characterized in that they consist of 40 to 55% by weight tin, 45 to 55% by weight zinc and 1 to 5% by weight cobalt.
 3. Ternary tin-zinc alloys according to claim 1, characterized in that they consist of 35 to 50% by weight tin, 50 to 65% by weight zinc and 0.1 to 5% by weight nickel.
 4. Ternary tin-zinc alloys according to claim 1, characterized in that they consist of 40 to 55% by weight tin, 40 to 60% by weight zinc and 1 to 8% by weight iron.
 5. Alkaline, neutral or weakly acidic galvanic electrolyte baths containing the alloy components in dissolved form and, optionally, other usual additives, for electrolytic production of alloy layers of ternary tin-zinc alloys according to claims 1 to
 4. 6. Process for producing alloy layers of ternary tin-zinc alloys according to claims 1 to 4, characterized in that they are deposited electrolytically from an alkaline, neutral or weakly acidic galvanic electrolyte bath containing the alloy components in dissolved form with, optionally, other usual additives.
 7. Use of galvanically produced alloy layers of ternary tin-zinc alloys according to claims 1 to 4 as anticorrosion layers.
 8. Use of galvanically produced alloy layers of ternary tin-zinc alloys according to claims 1 to 4, with subsequent passivation, as anticorrosion layers on ferrous materials.
 9. Use of galvanically produced alloy layers of ternary tin-zinc alloys according to claims 1 to 4 as weldable coatings.
 10. Use of galvanically produced alloy layers of ternary tin-zinc alloys according to claims 1 to 4 as final decorative coatings. 